1. Field of the Invention
The present invention concerns a frequency converter used in a microwave system, and more particularly the circuit layout structure of a frequency converter for supplying the ports for a quad diode mixer bridge.
2. Description of the Related Art
It is essential for a microwave system or radar system to include a frequency converter or mixer for converting an intermediate frequency (IF) signal into a radio frequency signal or vice versa. Such a frequency converter typically comprises diodes and a microstrip circuit layout for supplying the ports to the diodes.
In a prior art microwave mixer employing a balanced to unbalanced line converter (hereinafter referred to as a "balun") composed of microstrip lines and a slot line, two local oscillation signals with a phase difference of 90.degree. are supplied in an unbalanced form through the microstrip lines to a quad diode mixer bridge. The RF signal is supplied in a balanced form through another microstrip line to the quad diode mixer bridge. As the distance between the diodes arranged in the quad diode mixer bridge approaches zero, the variation of the image impedance becomes less so as to reduce the conversion loss due to the frequency variation. In addition, the transmission path for passing the signal from the frequency converter is orthogonal to prevent unwanted coupling.
However, such a frequency converter has an allowable frequency band which is limited to about one octave according to the frequency characteristics of a .lambda./4 open strip line a .lambda./4 short-circuited slot line. More specifically, assuming the frequency converter has a central frequency fo, the slot line is short-circuited at 2fo giving infinite admittance while the open strip line gives infinite impedance, so that the two local oscillation signals may not be supplied to the quad diode mixer bridge.
The following patents each disclose features in common with the present invention but do not teach or suggest the specifically recited frequency converter of the present invention: U.S. Pat. No. 5,428,840 to Sadhir, entitled Monolithic Double Balanced Microstrip Mixer With Flat Conversion Loss, U.S. Pat. No. 5,412,354 to Quan, entitled Single Layer Double Ring Hybrid Magic-Tee, U.S. Pat. No. 5,361,050 to Einbinder, entitled Balanced Split Ring Resonator, U.S. Pat. No. 4,980,925 to Blustine et al., entitled Monopulse First Detector Array, U.S. Pat. No. 4,876,744 to Endo et al., entitled Mixer With Rat Race Circuit, U.S. Pat. No. 4,823,400 to Dobrovolny, entitled Radio Frequency Mixer, U.S. Pat. No. 4,777,458 to Pardini, entitled Thin Film Power Coupler, U.S. Pat. No. 4,755,775 to Marczewski et al., entitled Microwave Balun For Mixers And Modulators, U.S. Pat. No. 4,636,755 to Gibbs, entitled High-Ratio, Isolated Microwave Branch Coupler With Power Divider, Phase Shifters, And Quadrature Hybrid, U.S. Pat. No. 4,509,209 to Itoh et al., entitled Quasi-Optical Polarization Duplexed Balanced Mixer, U.S. Pat. No. 4,498,061 to Morz et al., entitled Microwave Receiving Device, U.S. Pat. No. 4,420,839 to Hogerheiden Jr., entitled Hybrid Ring Having Improved Bandwidth Characteristic, U.S. Pat. No. 4,392,251 to Hallford, entitled Symmetric Microwave Mixer With Coplanar Diode Connection, U.S. Pat. No. 4,380,830 to Marchand et al., entitled Microwave Up-converter, U.S. Pat. No. 4,377,005 to Marchand et al., entitled Ultra-High Frequency Hybrid Mixer, U.S. Pat. No. 4,193,048 to Nyhus, entitled Balun Transformer, U.S. Pat. No. 4,099,126 to Hallford, entitled Frequency Converter Apparatus, U.S. Pat. No. 4,063,176 to Milligan et al., entitled Broadband High Frequency Mixer, U.S. Pat. No. 3,939,429 to Lohn et al., entitled Tunable High Frequency Input Circuit For A Television Receiver That Tunes Both VHF And UHF Channels And Can Be Readily Integrated, U.S. Pat. No. 3,818,385 to Mouw, entitled Hybrid Junction And Mixer Or Modulator, U.S. Pat. No. 3,803,519 to Ohi et al., entitled Wideband Directional Filter Device, U.S. Pat. No. 3,772,599 to Ernst et al., entitled Microwave Double Balanced Mixer, U.S. Pat. No. 3,659,206 to Hallford, entitled Microwave Balanced Mixer Circuit.